build:refactor arm_sdimage.sh as to no longer require root.
There where two reasons for needing root access while making ARM builds. The
first reason was that we needed root permissions to create device nodes and
assign permissions to files. The second reason we needed root was that we used
loop mounted files to copy the build output to the file backed storage.
The first problem is solved by building in unprivileged mode (adding -U) during
compilation. The second problem is solved by using mcopy to put files on the
fat partition and use the mkfs.mfs proto files to create MFS file system.
Future work might be to replace this script by the netbsd makefs tool.